It is customary in the oilfield industry to make up the above ground portion of an oilwell casing by attaching a tubing head adapter directly or via a tubing spool to the casing head. The tubing head adapter serves two purposes. First, it is threaded internally to accept tubing hangers and an optional back pressure control valve; it serves thereby to terminate the upper end of the production tubing, the innermost tubing within a producing oilwell. Secondly, it provides the initial flow control connections to the valves and plumbing which collectively make up the unit known as a christmas tree.
Above the tubing head adapter are found at least two major control master valves, usually called the bottom and top master valves. The purpose of the bottom master valve is to provide a backup shutoff mechanism in the event of a failure within the top master valve and a subsequent failure within the christmas tree structure. The bottom master valve is normally left in the open position. The top master valve is the main shutoff valve for the production oilwell and provides normally unrestricted flow to the christmas tree, being shut only when it is necessary to break open the christmas tree for some purpose.
Above the top master valve is found the christmas tree. This is a collection of chokes and valves necessary for metering and monitoring pressures, reducing pressures, diverting flows, and otherwise connecting a particular producing well to an overall network for distributing well product in a production operation.
A back pressure valve, inserted wtihin the tubing head, is used to prevent overflow in the event of removal or massive failure of the christmas tree, such as if the master valves are accidently knocked off. Its use creates significant problems, in that it is both difficult and time consuming to restore the flow of the producing well once the christmas tree has been repaired. A back pressure valve is an emergency, shut only device requiring skilled crews and special techniques for reopening it against significant and dangerous uphole flow pressures.